1. Field of the Invention
The present invention relates to a hydrogen supply device which supplies hydrogen, occluded in a hydrogen occluding alloy, to an apparatus which uses hydrogen, such as a fuel cell or a hydrogen engine.
2. Description of the Related Art
A great number of conventional systems have been developed for releasing hydrogen from a hydrogen occluding alloy, which hydrogen is occluded in, and supplying the hydrogen to a hydrogen-using apparatus, such as a fuel cell mounted in a moving body, such as a vehicle (e.g. Japanese Unexamined Patent Application, First Publications Nos. Sho 61-220009 and Hei 1-216024).
In the hydrogen occluding alloy, the input and output of heat accompanies the occlusion and release of hydrogen; when occluding the hydrogen, the heat must be discharged from the hydrogen occluding alloy; when releasing the hydrogen, heat must be supplied to the hydrogen occluding alloy. The amount of heat at this time is enormous, being 20 to 40 kJ per one mol of hydrogen. The amount of heat needed to release hydrogen is achieved by the heat capacity of the hydrogen occluding alloy, and as a result, the temperature of the hydrogen occluding alloy decreases.
Generally, the hydrogen dissociation pressure of the hydrogen occluding alloy increases as the alloy temperature rises, and when the temperature of the hydrogen occluding alloy decreases as the hydrogen is released, the hydrogen dissociation pressure (hereinafter abbreviated as “dissociation pressure”) drops. FIG. 4 shows dissociation pressure characteristics, the vertical axis representing dissociation pressure (absolute pressure) and the horizontal axes representing temperature (the top side expressed in degrees C., the bottom side expressed by a reciprocal of the absolute temperature T multiplied by one-thousand). In FIG. 4, the solid line and broken line represent two types of hydrogen occluding alloys having different dissociation pressure characteristics. The minimum release pressure represents the minimum dissociation pressure needed to supply hydrogen to the hydrogen-using apparatus, and when the dissociation pressure drops below the minimum release pressure, it becomes impossible to supply hydrogen. Therefore, when the temperature of the hydrogen occluding alloy has decreased during the release of hydrogen, the hydrogen occluding alloy must be heated to ensure that the dissociation pressure does not fall below the minimum release pressure. For this reason, conventional system generally provide a heat exchanger for the hydrogen occluding alloy.
When hydrogen is released from the hydrogen occluding alloy, the heat exchanger heats the hydrogen occluding alloy, and when occluding hydrogen in the hydrogen occluding alloy, the heat exchanger cools the hydrogen occluding alloy.
Conventionally, cooling water of a hydrogen-using apparatus, such as a fuel cell, is used as the heating medium of the heat exchanger. Since the cooling water, which has been used to cool the hydrogen-using apparatus, attains a high temperature, the cooling water is supplied as a heating medium to the heat exchanger, applying heat to the hydrogen occluding alloy while hydrogen is being released.
However, when cooling water is used as the heating medium for the heat exchanger, a cooling water circuit for feeding back the cooling water must be provided between the hydrogen-using apparatus and the hydrogen occluding alloy, making the device complex and large. Furthermore, the increase in the amount of cooling water being held consequently increases the weight of the device.